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From Flood to Drought?

In a complete turnaround from June, the “wet” areas of Minnesota have been few and far between so far in July.  Most of Southwest MN has averaged near 25% of the normal rainfall (or about 1.50 inches below normal) through Saturday the 19th.  Soil moisture has now declined to the point that, unless rain falls very soon, prospects will be deteriorating for some crops.

The week ending July 19th averaged about 8 degrees cooler than normal, bringing monthly departures to between three and five degrees below normal across Southwest MN.   The return of cooler than normal temperatures expected for the final weekend of July virtually guarantees our coolest July since 2009.

 

1st half July 2014 MN DFN Temps

 

Dry Spell Now at Three Weeks and Counting – July 14th

 

Though not as dry across the entire area as last week, all areas fell short of weekly normals during the July 7th to 13th time frame.   Cool weather will see us through this week without stressing developing crops too much, but we will really need some better rain as temperatures climb to near and above normal levels by July 20th and beyond.

7 day observed precip to 7-13-14

 

Last Week of Spring Excessively Wet

weekly rainfall to 6-22-14

A few locations south of Marshall received less than 2 inches of rain for the week ending at 7AM June 22nd.  However, amounts in excess of three inches were common across the listening area, and several areas to the northeast of Marshall measured well over 5 inches of rain.

 

Challenges of Forecasting Summer Rainfall

Posted Friday, June 6th —

Last Thursday, June 5th was an occasion when rainfall vastly exceeded forecasted amounts.  Thursday was particularly unique in that unexpectedly heavy rainfall affected the area not once but twice in the same day!  The morning rains developed in an atypical manner (more on that later), while rain had not been expected until much closer to mid-day.  In general, there are several possible causes of summer rains being more or less abundant than forecast.  Any or all of these can produce inaccurate forecasts of rainfall amounts.

  • Warm season rain events have more moisture to draw upon, and may be concentrated in small cells or pockets of intense rainfall.  This can produce an “all or nothing” type effect in which there can be a fine line between heavy rain and mere sprinkles, both in time and distance.
  • Formation of storms may occur at the boundary of warm, humid air and cooler “outflow” remaining from earlier storms.  These boundaries are often either undetected or handled poorly by the major forecast models. This effect definitely enhanced the strength of Thursday’s late afternoon thunderstorms.
  • Forecaster experience may not help overcome the lack of help from the forecast models, because a weather pattern which usually produces little to no rain may produce heavy rain in a small fraction of events.

Specifically, I have identified a handful of reasons that Thursday’s rains were unusual.

  • TIMING – Overnight storms typically die out around sunrise.  These storms developed just after sunrise.
  • SHORT-TERM MODELS had forecast the morning rains to stay to the north of the area.
  • OTHER STORMS were occurring at the same time over Arkansas and Missouri.  These often would have limited the moisture inflow needed to support heavy rainfall in southwest MN.
  • UPPER-LEVEL SUPPORT for the storms was weakening during the day.

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